The present invention relates to a grooving (groove cutting) work method and a cutting apparatus for forming a groove having a cross-section of a Christmas tree shape for fitting the blade legs of a turbine in the outer circumference of a turbine shaft.
A large sized and high power turbine is used in a high-efficiency combined cycle power generation plant. As a structure for fitting the impeller of such a turbine to a turbine shaft, there is known a structure in which turbine impeller blades 3 are fitted, one by one, in a number of tree-shaped grooves 2 having a cross-section of a Christmas tree shape formed in the outer circumference of a turbine shaft 1 as illustrated in FIG. 12 (refer to Japanese PCT National Publication No. 2004-507369).
FIG. 13 is a cross-sectional view illustrating a female tree-shaped groove in an enlarged manner. The tree-shaped groove has a symmetrical shape with respect to a groove center line S, in which the groove width is gradually tapered as a whole in the depth direction (in the downward direction of the drawing) while it is increased and decreased repeatedly. In the tree-shaped groove, three large-width portions 4, 5, and 6 each having a large width are arranged apart from each other in the groove depth direction. A large-width portion nearer to an opening portion 30 (formed at a shallower position) has a larger width, and large-width portion nearer to a bottom portion 31 (formed at a deeper position) has a smaller width. In the following description, as illustrated in FIG. 13, a constricted portion is referred to as a constricted portion 7, an inclined surface on the opposite side of the opening portion 30 (inclined surface facing the opening portion 30) is referred to as an incline surface portion 8, an outer circumferential portion is referred to as an outer circumferential portion 9, and an inclined surface on the opening portion 30 side is referred to as an undercut portion 10. In terms of quality, very strict accuracy is required for the distances of the undercut portion 10 in the groove depth direction between the large-width portions 4 and 5 and between the large-width portions 5 and 6.
A typical rotary cutting tool used in conventional tree-shaped groove cutting has a body obtained by integrally forming a shank portion and a blade portion. The blade portion has a shape in which conical portions and constricted portions are alternately arranged. A cutting blade portion is provided in the length area of each blade portion. The type of the cutting blade includes a straight blade and a right hand or left hand twisted blade which is excellent in cutting performance. In general, two to four cutting blades are provided.
In the above conventional rotary cutting tool, the difference in the diameter of the outermost portion between the constricted portion and the conical portion is large, which increases the difference in the cutting speed. Concerning the cutting speed, the optimum condition range is determined depending on the type of a cutting tool material and the type of a work material. If the cutting speed exceeds the condition range, an increase in tool wear occurs. For example, if cutting is carried out with the optimum cutting speed set in accordance with the outer diameter of the constricted portion, the cutting speed becomes higher at the conical portion, with result that the tool wear is increased.
Further, in the conventional rotary cutting tool, the contact length between the work material and the cutting blade is large, so that cutting resistance becomes large, making it easy to cause chatter. As a result, it is difficult to carry out cutting with a small milling machine having a low main shaft power and a low rigidity.